CAIRO— Egyptian judges and prosecutors struck back on Saturday against an edict by President Mohamed Morsi that gave him unchecked authority without judicial review, vowing to challenge his decree in court and reportedly going on strike in Alexandria.

Abdel Meguid Mahmoud, a prosecutor whom Mr. Morsi is seeking to fire, declared to a crowd of cheering judges at Egypt’s high court that the presidential decree was “null and void.” Mr. Mahmoud, who was appointed by Mr. Morsi’s predecessor, Hosni Mubarak, denounced “the systematic campaign against the country’s institutions in general and the judiciary in particular.”

Outside the court, the police fired tear gas at protesters who were denouncing Mr. Morsi and trying to force their way into the building.

The judicial backlash widened a power struggle over the drafting of a new constitution that has raised alarms about a return to autocracy 22 months after the ouster of Mr. Mubarak.

Mr. Morsi, the Islamist who became Egypt’s first elected president in June, is seeking to assert an authority unchecked by judicial review to forestall a court ruling expected on Dec. 2 that could disband the constitutional assembly and extend by two months the year-end deadline for that body to finish its work.

A high court dissolved an earlier assembly that was to draft a constitution last spring, and Mr. Morsi’s supporters accuse their secular opponents and judges appointed by Mr. Mubarak of trying to delay or derail the transition to democracy to prevent the Islamist majority from taking power.

The president’s opponents, in turn, accuse Mr. Morsi of seizing unchecked authority, noting that he holds executive and legislative power under a vague patchwork of interim constitutional declarations put in place by the military leaders who managed the first 18 months of Egypt’s post-Mubarak transition. The Supreme Constitutional Court dissolved the Islamist-dominated Parliament on the eve of Mr. Morsi’s election.

A council that oversees the judiciary on Saturday denounced Mr. Morsi’s decree, which was issued Thursday, as “an unprecedented attack” on its authority, and urged the president to retract the aspects of the decree circumscribing judicial oversight. State news media reported that judges and prosecutors walked out in Alexandria, and there were other news reports of walkouts in Qulubiya and Beheira, but those could not be confirmed.

In Cairo on Saturday, a coalition of secular opposition leaders and parties called for Mr. Morsi to withdraw his decree and disband the constituent assembly. The groups have long complained about the body’s domination by Islamists.

On Friday night their supporters set up a tent city for an open-ended sit-in in Tahrir Square, the center of the Egyptian revolt, and the groups have called for a demonstration there on Tuesday.

The Muslim Brotherhood, the Islamist group allied with Mr. Morsi, has called for demonstrations Sunday and Tuesday to support his moves as an effort to speed up Egypt’s transition to a constitutional democracy.

Near the square, a few hundred young men engaged in an unrelated battle with the police that has been going on for more than five days. They are demanding retribution against security officers who killed more than 40 people and blinded others with birdshot in clashes a year ago.

The protesters had hung a yellow banner across the street declaring “No Entry to the Brotherhood.” They blame the Muslim Brotherhood for failing to back them during last year’s protests.

On Saturday, most appeared unconcerned, if cynical, about Mr. Morsi’s decree, though some approved of his efforts to fire the Mubarak-appointed prosecutor and retry officials previously acquitted in the killings. “A drop of honey in a pool of poison,” said Hassan el-Masry, 19, who lost an eye during last year’s clashes.